Organometallic lead precursor, in-situ synthesis thereof, lead-titanium based thin film using the same, and preparation method therefor

ABSTRACT

An organometallic lead precursor, represented by following formula: 
     
         L.sub.x ·Pb(THD).sub.2                             I! 
    
     wherein L is an electron donor ligand selected from the group consisting of NR 3  (R=H, CH 3 ) gas and Cl 2  gas; THD denotes 2,2&#39;,6,6&#39;-tetramethyl-3,5-heptanedione; and x is in the range of 0.5 to 2, is prepared by flowing a gas phase electron donor into a bubbler containing bis (2,2&#39;,6,6&#39;-tetramethyl-3,5-heptanedione)Pb at a predetermined temperature, to synthesize, in-situ, an adduct. The precursor exhibits a remarkable improvement in volatility and in stability at the vaporization point. 
     Lead-titanium based thin films prepared from the precursor, display superior reproducibility and reliability.

This is a divisional of copending application Ser. No. 08/539,518 filedon Oct. 5, 1995.

BACKGROUND OF THE INVENTION

1. Field of the invention

The present invention relates, in general, to a novel organometalliclead precursor and its in-situ synthesis and, more particularly, to anorganometallic lead precursor with an improvement in volatility. Also,the present invention is concerned with a lead-titanium based thin filmusing the same and with a preparation method for preparing the same.

2. Description of the Prior Art

PbTiO₃ thin film or its related thin films (hereinafter referred to as"PT thin films"), used as ferroelectric thin films for semiconductorcapacitor, are prepared from organometallic precursors. For productionof the PT based thin films, there is generally used a metal-organicchemical vapor deposition process (hereinafter referred to as "MOCVD"),a technique for growing thin layers of compound semiconductors. Inpractice, MOCVD comprises preheating a liquid or solid of organometallicprecursor to vaporize it and decomposing the vapor by heat or plasma, todeposit a thin film.

It is necessarily required that appropriate organometallic precursors beselected in order to obtain desired properties and quality of the PTbased thin film. The following are general conditions thatorganometallic precursors should have: first, organometallic precursorsshould be able to be easily vaporized and deposited at low preheattemperatures, in addition to being stable at the preheat temperatures;second, the difference between the vaporization temperature and thedecomposition temperature should be large enough; third, it is preferredthat the organometallic precursors are not decomposed or changed by themoisture contained in air; finally, they should not be environmentallytoxic.

Among the organometallic precursors useful for preparing the PT basedthin films, representative are organometallic Ti precursors andorganometallic Pb precursors. The former has extensively been researchedand developed by virtue of its ability to vaporize at relatively lowtemperatures. On the other hand, most of the organometallic leadprecursors have problems in that they are extremely harmful to humanbody, and have poor in thermal stability.

Bis(2,2',6,6'-tetramethyl-3,5-heptanedione)Pb (hereinafter referred toas "Pb(THD)₂ "), an organometallic lead precursor, exhibits encouragingcharacteristics, such as low toxicity and high moisture stability. It isthus, most widely used. However, this organometallic lead precursorshows a crucial problem in that the difference between its vaporizationtemperature and decomposition temperature is not large. That is to say,Pb(THD)₂ is thermally instable at around 150° C., a typical temperatureat which solid Pb(THD)₂ overcomes its intermolecular interaction and isvaporized.

Referring to FIG. 1, Pb(THD)₂ isotherms show weight loss with the lapseof time. A TGA balance is utilized for measuring the weight loss. Attemperatures lower than 120° C., the weight of Pb(THD)₂ decreasessteadily. However, at temperatures higher than 120° C., the weightdecrease is not observed after a certain time interval. It is believedthat Pb(THD)₂ is thermally stable only up to 120° C., and thermallyinstable above 120° C., such that it decomposes and/or forms anonvolatile chemical species.

As described previously, a temperature of 140° to 160° C. is required tovaporize Pb(THD)₂ to the degree that it is applicable to MOCVD, butvaporization and decomposition of Pb(THD)₂ occurs simultaneously at 130°C. or more. In practice, when Pb(THD)₂ is used to deposit PT based thinfilms, decomposition of the precursor occurs in the bubbler and thus,the bubbler must be charged with fresh precursor after several runs.Accordingly, it is virtually impossible to prepare a thin film withPb(THD)₂, in a large quantity and in uniform quality. Consequently,despite low toxicity and high moisture stability, Pb(THD)₂ has greatdifficulties in its application to Pb MO(metal organic) sources becauseof its low volatility and poor thermal stability.

SUMMARY OF THE INVENTION

Therefore, a primary object of the present invention is to overcome theabove problems encountered in prior art and to provide a novelorganometallic lead precursor, improved in volatility and in stabilityat its vaporizing point.

It is another object of the present invention to provide a method forsynthesizing the organometallic lead precursor by a in-situ process.

It is a further object of the present invention to provide alead-titanium thin film made of an organometallic lead precursor, whichis superior in reliability and reproducibility.

Based on intensive and through research and study by the presentinventors, the above objects are accomplished by providing anorganometallic lead precursor for ferroelectric lead, titanium basedthin film, represented by the following Formula I:

    L.sub.x ·Pb(THD).sub.2                             I!

wherein L is an electron donor ligand selected from the group consistingof NH₃, N(CH₃)₃, and Cl₂ gas; THD denotes2,2',6,6'-tetramethyl-3,5-heptanedione; and x is in the range of 0.5 to2.

In accordance with another aspect of the invention, there is provided amethod for the preparation of organometallic lead precursor, comprisingflowing an electron donor in gas phase into a bubbler containing bis(2,2',6,6'-tetramethyl-3,5-heptanedione)Pb at a predeterminedtemperature, to synthesize, in-situ, an adduct represented by the aboveformula.

In accordance with a further aspect of the invention, there is provideda method for the deposition of lead-titanium based thin film, comprisingthe steps of: flowing an electron donor in the gas phase into a bubblercontaining bis (2,2',6,6'-tetramethyl-3,5-heptanedione)Pb at a selectedreaction temperature, to synthesize, in-situ, an adduct represented bythe above formula; cooling the adduct to solidify it; heating thebubbler to a temperature lower than said reaction temperature, tovolatilize the adduct; passing the adduct with a carrier gas into ametal-oxide vapor deposition reactor; and reacting the volatilizedadduct with a titanium precursor source at a high temperature and atreduced pressure in an oxidative atmosphere by metal-organic chemicalvapor deposition.

BRIEF DESCRIPTION OF THE DRAWINGS

The above objects and other advantages of the present invention willbecome more apparent by describing in detail the preferred embodiment ofthe present invention with reference to the attached drawings in which:

FIG. 1 shows Pb(THD)₂ isotherms illustrating weight loss with the lapseof time;

FIG. 2 is a schematic longitudinal section showing bubbler equipmentuseful to synthesize an organometallic lead precursor of the presentinvention by an in-situ process; and

FIG. 3 is a schematic view showing an MOCVD apparatus useful forpreparing a lead-titanium based thin film from the organometallic leadprecursor of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

In a Pb(THD)₂ molecule, Pb(II), the central atom, is coordinately bondedwith THD, the ligands, by incorporation of the electrons on the carbonylgroups of the ligands into the bonding sphere of the metal. Generally,THD, which is a neutral ligand without nominal charge, cannot donatesufficient electrons to the Pb atom. Accordingly, the electrophilic Pbatom comes to attractively interact with the THD ligands of neighboringPb(THD)₂ molecules. As a result, Pb(THD)₂ molecules form a stable solidstructure at room temperature.

on the other hand, coordination bonding of the Pb atom with additionalligands capable of donating sufficient electrons results in weakening orbreaking the intermolecular interaction in the solid structure. Thecoordinated Pb(THD)₂ molecules are able to volatilize at lowertemperatures than original Pb(THD)₂.

In accordance with the present invention, NH₃, N(CH₃)₃, or Cl₂ moleculesare utilized as the additional electron donor. Since the NH₃ or Cl₂molecule has lone electron pairs, it can supply sufficient electrons tothe metal atom. In addition, such an additional electron donor isrelatively small in size, so that it can easily contact with the Pbatom. Therefore, the NH₃, N(CH₃)₃, or Cl₂ electron donor itself can bechemically adducted to Pb(THD)₂, as represented by the followingreaction formulas:

    x NR.sub.3 (g)+Pb(THD).sub.2 (s)→(NR.sub.3).sub.x ·Pb(THD).sub.2 (s)

    y Cl.sub.2 (g)+Pb(THD).sub.2 (s)→(Cl.sub.2).sub.y ·Pb(THD).sub.2 (s)

wherein R is H or CH₃ ; x is in the range of 0.5 to 2; and y is in therange of 0.5 to 2. In the NH₃, N(CH₃)₃, or Cl₂ adduct, the additionalelectron donors break the intermolecular interaction among Pb(THD)₂molecules, so that the adduct can be volatilized at low temperatures.The obtained adduct can be utilized as an organometallic lead precursorwith a remarkable improvement in volatility.

In accordance with the present invention, the organometallic leadprecursor is prepared by a method comprising flowing a gas phaseelectron donor into a bubbler containing Pb(THD)₂ at a predeterminedtemperature, to synthesize, in-situ, an adduct represented by thefollowing formula:

    L.sub.x ·Pb(THD).sub.2

wherein L is an electron donor ligand selected from a group consistingof NH₃, N(CH₃)₃, and Cl₂ ; and

x is in the range of 0.5 to 2.

The gas phase electron donor used to prepare the organometallic leadprecursors of the present invention is selected from the groupconsisting of NH₃, N(CH₃)₃, and Cl₂.

A more detailed description for the preparation of the presentorganometallic lead precursors will be given as follows.

For the preparation of (NR₃)_(x) ·Pb(THD)₂ (R=H or CH₃) by an in-situprocess, solid phase Pb(THD)₂ is placed in a bubbler, and NR₃ (R=H orCH₃) gases are used as a carrier gas. The temperature of the bubbler iskept to 100° to 120° C. Pb(THD)₂ in the bubbler reacts with NR₃ (R=H orCH₃) carrier gas, and (NR₃)_(x) ·Pb(THD)₂ (R=H or CH₃, 0.5≦x≦2) issynthesized. The produced chemical species are so volatile that they arevaporized instantly at the bubbler temperature of 100° to 120° C.Vaporized (NR₃)_(x) ·Pb(THD)₂ is carried by unreacted electron donorgases (NH₃ or N(CH₃)₃), and can be used for the thin film deposition inthe reactor. Consequently, by simply flowing NR₃ (R=H or CH₃) as carriergas, Pb(THD)₂ precursor can be easily vaporized at a temperature of 100°to 120° C. This bubbling temperature is about 40 degrees lower than thatof conventional processes (using Pb(THD)₂ as precursor and Ar or N₂ ascarrier gas). Therefore, there is no decomposition of precursor duringthe bubbling process and a constant vapor pressure can be maintainedover an elapsed bubbling time. (NR₃)_(x) ·Pb(THD)₂ synthesized by thein-situ process was analyzed by an Element Analysis Technique. It wasascertained that the value of x is 0.5˜2.

For preparation of (Cl₂)_(x) ·Pb(THD)₂, Pb(THD)₂ of solid phase isplaced in the silica boat as shown in FIG. 2 and then, an electron donorin the gas phase (Cl₂) is supplied under the control of a Mass Flowcontroller. At an elevated temperature, an adduct between Pb(THD)₂ andCl₂ is synthesized in the gas phase. As for the reaction conditions,where Cl₂ gas flows through the tube, the temperature of Zone 1 ismaintained in the range of about 130° to about 150° C. At thistemperature range, Cl₂ gas reacts with Pb(THD)₂ rapidly. Thereafter, thesynthesized gas phase adduct ((Cl₂)_(x) ·Pb(THD)₂) is solidified in Zone2 because the wall of the tube in Zone 2 is cooled to room temperature.Since Cl₂ gases are instantly reacted with Pb(THD)₂ under theseconditions, the amount of synthesized adduct can be calculated on thebasis of the amount of charged Pb(THD)₂ in Zone 1. Meanwhile, unreactedelectron donor gas is not passed into a reactor connected to the tube ofZone 2, but drained through a bypass and removed through reaction withan aqueous NaOH solution. Then, the carrier gas is replaced with N₂ andthe bubbler temperature is adjusted to 80° to 110° C., which is 40 to 80degrees lower than the temperature of the conventional process.

Referring now to FIG. 3, there is shown an MOCVD apparatus useful todeposit PT based thin films. As shown in this figure, this MOCVDapparatus, which is a kind of a vacuum CVD apparatus operating in a hotwall manner, is provided with bubbler equipment for precursor sources,e.g. a Pb source and a Ti source; a reactor; and a cold trap.

For the deposition of a PT based thin film by using NR₃ (R=H, CH₃)adducted Pb(THD)₂, Pb(THD)₂ is first charged into a bubbler. Then, thetemperature of bubbler is adjusted to 100°˜120° C. and NR₃ gas with aflow rate of 50˜100 sccm is employed. The in-situ generated (NR₃)_(x)·Pb(THD)₂ is carried to a reactor by the NR₃, which is not consumed inthe reaction with Pb(THD)₂. For the Ti source, Ti(O--CH₂ CH₃)₄(hereinafter referred to as "Ti(OEt)₄ ") is typically used, and it isheated to 100°˜125° C., while N₂ gas is preferably used as a carriergas.

For the deposition of a PT based thin film by using chlorine-adductedPb(THD)₂, the prepared (Cl₂)_(x) ·Pb(THD)₂ in Zone 2 of the bubbler ofFIG. 2 is first heated to 80°˜110° C. In order to carry the vaporizedchlorine-adducted Pb(THD)₂, N₂ gas with a flow rate of 50˜100 sccm isemployed. In the same way, Ti(OEt)₄ is used as a Ti source, and 10˜20sccm of N₂ gas can be used as a carrier gas. Thereafter, the vaporizedprecursor sources are decomposed in the reactor heated to a hightemperature, for example, about 600° to about 700° C. and then, reactedwith oxygen gas which is provided from another regulated route, fordeposition of PT based thin film on a substrate. For PbTiO₃ thin film,the oxygen gas is flowed in a rate of 500 sccm. The cold trap which isconnected with the reactor, in the meanwhile, removes unreactedprecursor sources. A vacuum pump works to maintain a reduced pressure inthe reactor.

The preferred embodiments of the present invention will now be furtherdescribed with reference to specific examples.

EXAMPLE 1

The MOCVD apparatus of FIG. 3 was used to deposit a PbTiO₃ thin film.1.0 g of Pb(THD)₂ was charged in a stainless tube of the bubbler andheated to about 110° C. Separately, 10 g of Ti(OEt)₄ was charged inanother bubbler and heated to about 115° C. The Ti source was suppliedto the reactor by a carrier gas of N₂ with a flow rate of about 15 sccm,whereas the Pb source was supplied by a carrier gas of NH₃ with a flowrate of 50 sccm. Then, the reactor was heated to about 600° C., todeposit the PbTiO₃ thin film.

More detailed reaction conditions are reported in the following Table I.

                  TABLE I                                                         ______________________________________                                        Deposition Conditions for PbTiO.sub.3                                                          Reaction Condition                                           Item               Pb source                                                                              Ti source                                         ______________________________________                                        Precursor          Pb(THD).sub.2                                                                          Ti(OEt).sub.4                                     Temp. in Bubbler   110° C.                                                                         115° C.                                    Carrier Gas        NH.sub.3 N.sub.2                                           Flow Rate of Carrier Gas                                                                         50 sccm  15 sccm                                           Oxygen Flow Rate   500 sccm                                                   Temp. & Pressure in Reactor                                                                      600° C., 2.0 torr                                   ______________________________________                                    

Under the conditions given in Table I, the reactor was operated for 30minutes, to obtain a PbTiO₃ thin film of perovskite phase with athickness of 300 nm.

EXAMPLE II

(Cl₂)_(x) ·Pb(THD)₂ was prepared using the bubbler equipment of FIG. 2.For this, a quartz boat containing 1.5 g of Pb(THD)₂ was positioned inZone 1. Zone 1 was heated to 140° C. and Zone 2 was maintained in roomtemperature by circulating cold water around Zone 2. Dry chlorine gasflowed in a rate of 50 sccm by 20 minutes and was consumed by reactionwith Pb(THD)₂ positioned in Zone 1. The gas phase adduct thus produced,(Cl₂)_(x) ·Pb(THD)₂, was solidified in Zone ₂, cold region.

Thereafter, Example 1 was repeated using the deposition conditions givenin the following Table II. Particularly, the carrier gas for the Pbsource was replaced by nitrogen gas in this Example.

                  TABLE II                                                        ______________________________________                                        Deposition Conditions for PbTiO.sub.3                                                         Reaction Condition                                            Item              Pb source  Ti source                                        ______________________________________                                        Precursor         Cl.sub.X.Pb(THD).sub.2                                                                   Ti(OEt).sub.4                                    Temp. in Bubbler  95° C.                                                                            115° C.                                   Carrier Gas       N.sub.2    N.sub.2                                          Flow Rate of Carrier Gas                                                                        50 sccm    15 sccm                                          Oxygen Flow Rate  500 sccm                                                    Temp. & Pressure in Reactor                                                                     600° C., 2.0 torr                                    ______________________________________                                    

Under such conditions, the reactor was operated for 30 minutes, toobtain a PbTiO₃ thin film of perovskite phase with a thickness of 350nm.

Although the Examples illustrate the practice of the present invention;using the organometallic lead precursors (NR₃)_(x) ·Pb(THD)₂ where R isH or CH₃ and (Cl₂)_(x) ·Pb(THD)₂, for deposition of PbTiO₃ thin film,these precursors are believed to provide similar results when other PTbased thin films are prepared therefrom.

The following are the advantages obtained when the organometallic leadprecursors of the present invention, (NR₃)_(x) ·Pb(THD)₂ and (Cl₂)_(x)·Pb(THD)₂, are used for PT based thin films.

First, the above precursors can be used repetitively in a bubblerwithout any decomposition. So, it is possible to obtain a PT based thinfilm superior in reliability and reproducibility.

Second, the precursors of the present invention are able to be vaporizedat much lower temperatures than is Pb(THD)₂.

Third, the precursors of the present invention are very stable at thepreheat temperatures for vaporization.

Fourth, since (NH₃)_(x) ·Pb(THD)₂ and (Cl₂)_(x) ·Pb(THD)₂ arehygroscopic, there is a danger that these precursors decompose duringtreatment if they are prepared outside the bubbler and then deliveredinto the bubbler. In the present invention, the above danger iseliminated because the precursors are synthesized in the bubblerin-situ.

Finally, N₂ gas, which is utilized as a carrier gas for (Cl₂)_(x)·Pb(THD)₂ precursor prevents corrosion or damage to the reactor.

Other features, advantages and embodiments of the invention disclosedherein will be readily apparent to those exercising ordinary skill afterreading the foregoing disclosures. In this regard, while specificembodiments of the invention have been described in considerable detail,variations and modifications of these embodiments can be effectedwithout departing from the spirit and scope of the invention asdescribed and claimed herein.

The expression "PT based thin film", or "lead titanium based thin film",as used in the specification denotes PbTiO₃, Pb(ZrTi)O₃ (PZT) and dopedPb(ZrTi)O₃ (PZT), such as Pb(NbZrTi)O₃ (PNZT), Pb(LaZrTi)O₃ (PLZT),Pb(TaZrTi)O₃ (PTZT) and Pb(ScZrTi)O₃ (PSZT).

What is claimed is:
 1. An organometallic lead precursor forferroelectric lead-titanium based thin films represented by thefollowing Formula I:

    L.sub.x ·Pb(THD).sub.2

wherein L is an electron donor ligand selected from the group consistingof NR₃ gas, wherein R is hydrogen or methyl, and Cl₂ gas; THD denotes2,2',6,6'-tetramethyl-3,5-heptanedione; and x is in the range of 0.5 to2.
 2. An organometallic lead precursor for ferroelectric lead-titaniumbased thin films in accordance with claim 1, wherein said organometalliclead precursor is volatilized at a temperature of about 80° to about120° C.
 3. An organometallic lead precursor for ferroelectriclead-titanium based thin films in accordance with claim 2, wherein saidorganometallic lead precursor is volatilized at a temperature of about80° to about 110° C.
 4. An organometallic lead precursor forferroelectric lead-titanium based thin films in accordance with claim 2,wherein said organometallic lead precursor is volatilized at atemperature of about 100° to about 120° C.
 5. A method for thepreparation of an organometallic lead precursor, comprising flowing agas phase electron donor into a bubbler containing bis(2,2',6,6'-tetramethyl-3,5-heptanedione)Pb at a predeterminedtemperature, to synthesize, in-situ, an adduct represented by thefollowing formula II:

    L.sub.x ·Pb(THD).sub.2

wherein L is an electron donor ligand selected from the group consistingof NR₃, wherein R is hydrogen or methyl, and Cl₂ ; THD denotes2,2',6,6'-tetramethyl-3,5-heptanedione; and x is in the range of 0.5 to2;said gas phase electron donor serving as a carrier gas and beingselected from the group consisting of NR₃, wherein R is hydrogen ormethyl, and Cl₂.
 6. A method in accordance with claim 5, wherein saidpredetermined temperature is from about 140° to 160° C. and said gasphase electron donor is NR₃, wherein R is hydrogen or methyl.
 7. Amethod in accordance with claim 5, wherein said predeterminedtemperature is from about 130° to 150° C. and said gas phase electrondonor is Cl₂.